Transformation of Imine Cages into Hydrocarbon Cages

Article abstract of DOI:10.1002/anie.201814243

In contrast to organic cages which are formed by exploiting dynamic covalent chemistry, such as boronic ester cages, imine cages, or disulfide cages, those with a fully carbonaceous backbone are rarer. With the exception of alkyne metathesis based approac

Full text of DOI:10.1002/anie.201814243

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Accepted Article
Title: Transformation of Imine Cages to Hydrocarbon Cages
Authors: Michael Mastalerz, Tobias HG Schick, Jochen C Lauer, and
Frank Rominger
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Transformation of Imine Cages to Hydrocarbon Cages
Tobias H. G. Schick,§,[a] Jochen C. Lauer,§,[a] Frank Rominger[a] and Michael Mastalerz*[a]
consecutive steps with overall yields of < 0.1% in both cases.
Abstract: In contrast to organic cages which are formed by exploiting
dynamic covalent chemistry, such as boronic ester cages, imine
cages or disulfide cages, those with a fully carbonaceous backbone
are rarer. With the exception of alkyne metathesis based approaches,
the vast majority of hydrocarbon cages needs to be synthesized by
kinetic bond formation. This implies a multiple step synthesis and no
correction mechanism in the final macrocyclization step - both
responsible for low overall yields. Whereas for smaller cages the
intrinsic drawbacks are not always obvious, larger cages are seldom
synthesized in yields beyond a few tenth of a percent. Here we
present a three-step method to convert imine cages into hydrocarbon
cages. This has been successfully applied even for larger structures
such as derivatives of C72H72 – an unknown cage suggested by Fritz
Vögtle more than 20 years ago.
Among the suggested structures was also a larger ‘cubic’ C72H72
with eight benzene rings connected via ethylene bridges (Fig. 1a),
of which the synthesis has never been reported to date. Similar to
how the tetrahedral C36H36 is a cut-out structure of fullerene C60,
the C72H72 is related to a cubic C120 derivative containing six
cyclooctatetraene rings (Fig. 1c).[10]
a)
The interest in organic cages has raised in recent years not least
because some of them became accessible in good yields from
rather simple building blocks by applying dynamic covalent
chemistry (DCC) reactions such as the formation of imines from
amines and aldehydes,[1] boronic esters from boronic acids and
diols,[2] or disulfides from thiols,[3] to name a few.[4] All cages
generated by these reactions have in common that the
constructive elements are typically based on bonds with at least
one heteroatom. In contrast, cages with pure carbon atom
backbones are rarer and, with a few exceptions by exploiting the
reversibility of alkyne metathesis,[5] needed to be synthesized by
irreversible formation of carbon bonds.[6] One of the earliest
examples of a hydrocarbon cage synthesis is the approach of Olof
Wennerström et al.:[7] In a one-pot reaction the 1,3,5-triformyl
benzene was undertaken a sixfold Wittig reaction with 1,3-
bisbenzylphosphonium bromide giving the helical olefin cage 1 in
less than 2% isolated yield (Fig. 1a, top). The follow-up reductive
transformation to the ethylene-bridged compound 2 was nearly
quantitative. Ten years later, Vögtle et al. presented the seven-
step synthesis of a tetrahedral hydrocarbon cage C36H36 where
four benzene rings are connected with ethylene units (Fig. 1a,
bottom).[8] The overall yield of this route was about 3%, because
the limiting step is the final macrocyclization or cage formation,
which was achieved in only 11% although high-dilution conditions
have been applied. Nevertheless, as Vögtle et al. have pointed
out, the scaffold of this hydrocarbon cage is a cut-out structure of
fullerene C60 and therefore attractive as a potential precursor to
synthesize that (Fig. 1b). In follow-up contributions larger related
structures have been proposed and some of them (a C54H48 and
a C60H60) have been synthesized,[9] again in ten to fourteen
b)
c)
Figure 1. a) Reported syntheses of helical cage 1[7] and tetrahedral cage
C36H36 and the unknown structure C72H72, suggested by Vögtle et al.[9] b)
[8]
Scaffold of tetrahedral cage C36H36 and fullerene C60 for comparison. c) Scaffold
of cubic cage C72H72 and fullerene C120 for comparison.
Although the synthetic transformation of C72H72 to C120 is very
challenging, the latter is an attractive compound making it
worthwhile to work on new methods to generate such ethylene-
bridged hydrocarbon cages in shorter and thus more efficient
synthetic routes, which will be presented herein. In classical
cyclophane chemistry an often used strategy is the late-stage ring
contraction under elimination or extrusion of small heteroatomic
molecules to generate the ethylene bridges; even to make
strained cyclophanes. Among the possibilities to realize the C-C-
bond formation is the extrusion of nitrosamines, developed by
Overberger et al.,[11] as has been reported by Takemura et al. to
[a]
Tobias H. G. Schick,§ Jochen C. Lauer,§ Frank Rominger and
Michael Mastalerz*
Organisch-Chemisches Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
*E-mail: michael.mastalerz@oci.uni-heidelberg.de
§both authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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form [2.2]-meta or [2.2]-para cyclophanes.[12] We investigated this
reaction to convert imine cages via the nitrosamine cages to
hydrocarbon cages.
a)
First target was the transformation of the [2+3] imine cage 5a[11]
to Wennerström type hydrocarbon cage 8a (Fig. 2). Imine cage
5a was synthesized in 83% from the reaction of triamine 3 and
isophthaldehyde 4a.[11a] The follow-up reduction to amine cage 6a
occurred in 93 % yield by using NaBH4 as reducing agent.[11a]
Exhaustive nitrosylation of the secondary amine functions could
either be achieved with sodium nitrite and hydrochloric acid or
neat isoamyl nitrite, giving 48% or 68% yield of nitrosylated cage
7a, respectively. The analytics of the transformation to this
nitrosamine cage 7a is restricted to IR spectroscopy, DOSY NMR
spectroscopy, mass spectrometry and elemental analysis.
1
Although H and 13C NMR spectra of the product looked always
very similar, such as fingerprints (see Fig. 2b and Supporting
Information), no reasonable peak assignment can be done,
because for each nitrosamine group there exist two thermally
stable E/Z isomers due to a partial N-N double bond character.[12]
Therefore, by nitrosamine formation there are 26 = 64 different
1
possible isomers for 7a, which explains the complicated H and
b)
13C NMR spectra. Investigations by DOSY NMR revealed for all
peaks the same diffusion coefficient (D = 9.16 m2/s), thus all
isomers have a very similar solvodynamic radius (rS = 0.58 nm),
which is expected (see Supporting Information). Nitrosamine
formation could be clearly found by two bands at 1443 and 1134
cm-1 in the IR-spectrum.[13] Furthermore, by mass spectrometry a
peak at m/z = 101.512 was detected, fitting the expected m/z ratio
of 101.513 for a six-fold nitrosylation (see Supporting Information).
Furthermore, the elemental analysis is in the expected range of
specification. The final step of the transformation of the imine
cage to a fully hydrocarbon cage is the reductive elimination of
the nitrosylamine unit under ring-contraction, which was
performed by using the original conditions (EtOH, NaOHaq,
Na2S2O4, reflux) of Overberger et al..[14] Besides the desired
hydrocarbon cage 8a (25% yield), the mono nitrosamine cage 9
was isolated in 20% yield. However, any attempts to perform the
ring contraction on this compound by applying the same or similar
conditions gave only minor transformation (about 20%, see Fig.
S98). This implies that the absolute E/Z-configuration of the
nitrosamine unit plays a yet underestimated role in the reaction
mechanism, which was postulated to occur via the formation of a
diazene (or N-nitrene) as intermediate and therefore would not be
affected by the stereochemistry of the nitrosamine groups.[14]
However, more detailed studies need to be done to get further
mechanistic insights. The overall yield of the three-step method is
13%, thus approx. ten times higher than for the reported two-step
approach of Wennerström et al.[7] Other [2+3] imine cages, such
as pyridine cage 5b[11b] can be transformed within three steps in
the same way to the corresponding hydrocarbon cage 8b in yields
of 50% (overall 22%) and even the one with sterically more
demanding methoxy-groups in the interior (5c) was transformed
by ring contraction in 20% in the last step to 8c (Fig. 2a). The
much higher yields of the Overberger rearrangement of pyridine
cage 7b give a hint that electronic demand of the aromatic rings
may stabilize the intermediates. However, to further elucidate this,
more diverse imine cages with additional substituents need to be
investigated.
6a
7a
8a
c)
Figure 2. a) Synthesis of Wennerström-type carbon cages 8a-8c; i) MeOH,
room temp., 2d (8a,8b) or 12 h (8c); ii) NaBH4, MeOH, 12 h; iii) isoamyl nitrite,
50 oC (8a,8b), or 60 oC (8c) 12 h; iv) Na2S2O4, NaOHaq (20%), EtOH, reflux 12
h. b) 1H NMR comparison of amine 6a, nitrosamine 7a and CC-cage 8a in CDCl3
(300 MHz, room temp.). c) interconversion of D3-symmetric helical conformers
of 8a via the achiral D3h transition.
All small hydrocarbon cages 8a-8c and the mono nitrosamine
cage 9 have been characterized by single-crystal X-ray diffraction
(Fig. 3, for crystallographic details see Supporting Information).
Hydrocarbon cage 8a crystallizes as racemate of two helical
conformers along with two molecules of CHCl3 in the unit cell P-
1. All ethyl-chains are pointing outside the cage, which is
expected due to the high rotation barrier of these found in an
alternating fashion.[15] The ethylene bridges are arranged in a
gauche conformation with torsional angles between 76.7º and
88.1º. The distance between the two coplanar benzene rings on
top and bottom is 6.1 Å, the distances between the inner aromatic
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hydrogen atoms are about 3.2 Å - too small to host guest
molecules. Cage 9 crystallized also as racemic mixture of the two
helical conformers. It is worth mentioning that the NNO, which is
conformational fixed is pointing with the oxygen towards the
threefold ethyl-substituted ring. The two other cages (8b and 8c)
are of lower symmetry (C1) in the solid state (Fig. 3 and the
Supporting Information).
We were interested, if this method allows the synthesis of even
larger structures, such as derivatives of the unknown C72H72
depicted in Fig. 1.[9b] Recently we published the formation of
truncated tetrahedral [4+4] imine cages, such as 10, which
contains already the eight aromatic rings necessary for this
compound.[17] Reduction of imine cage 10 to the amine 11 with
NaBH4 was quantitative as well as the nitrosylation to 12 (Fig. 4a).
After performing the Overberger rearrangement, hydrocarbon
cage 13, which is a dodecakis-ethyl-substituted derivative of
previously unknown C72H72, was isolated in 5% yield. Considering
that this is a 12-fold reaction, the average yield per CC-bond
formation is 77%; exactly what has been reported by Overberger
in his original contribution.[14] Including the imine cage formation,
cage 13 was accessible from rather simple precursors in only four
steps with an overall yield of 1.4% yield. Although no direct
comparison to a reported route exists, one can assume that if it
would have been synthesized in a similar way than e.g. C54H54 it
would have been many more steps and most likely with overall
yields less than tenth of a thousandth.
a)
a‘)
b)
b‘)
Cage 13 crystallizes in the tetragonal unit cell I-4 with a
flattened molecule of S4 symmetry. As for the smaller cages 8a-
8c and 9, all ethyl groups are pointing outwards of the molecular
structure. Here, three different conformational orientations of the
ethylene bridges can be found, one that is anti-periplanar with
172.7º, and two synclinal oriented with torsional angles of 48.6º
and 86.2º. The 1H NMR spectrum of 13 show a number of broad
peaks at room temperature, however when cooling down to 253
K only sharp peaks can be detected (Fig. 4b). In the aromatic
region, three distinct peaks with the same integrals are resonating
at δ = 6.95, 6.85, and 5.76 ppm. This is exactly the number of
different peaks that are expected for the S4-symmetric molecular
structure found in the X-ray crystal structure (Fig. 4c and d). A
closer look at the structure reveals one inner aromatic proton Hi
pointing towards the ethyl groups and two outer ones, whereas
one (Ho1) is located over the centre of the adjacent ethyl
substituted ring in a distance of approx. 3.6 Å, thus experiencing
the anisotropic ring current of this one, which explains the up-field
shift to 5.76 ppm (Fig 4b). Upon heating the NMR sample, only
the two aromatic peaks Ho1 and Ho2 start to broaden and coalesce
at 303 K (kc =1057 Hz, ΔG = 57 kJ/mol), which can be explained
by a twisting movement of the two disc-like parts of the molecule
against each other (Fig. 4e).
c)
c‘)
d)
d‘)
Figure 3. Solid state structures of cages 8a (a: side view and a’: top view), 9 (b
and b’), 8b (c and c’), and 8c (d and d’), determined by single-crystal X-ray
diffraction. Note: hydrogen atoms and solvate molecules are omitted for clarity.
Depicted is only one enantiomer of the racemic mixture, each. For
crystallographic details, see Supporting Information.
Finally, [4+6] tetrahedral imine cage 14[18] was converted within
three steps to an even larger C84H84 derivative 17 (Fig. 5a), again
with 5% yield in the twelvefold Overberger reaction, which is the
same as for smaller cage 13. The 1H NMR spectrum of cage 17
at room temperature shows only a few sharp distinct signals,
which is a hint for a faster dynamic interconversion of conformers.
1
By temperature-dependent H NMR spectroscopy, the inversion
barrier for cage 8a between the helical D3h-symmetric conformers
via the D3-symmetric achiral transition state (see Fig. 2c) has
been determined by monitoring the coalescence of the two peaks
of the ethylene unit. The coalescence occurs at Tc = 263 K, which
corresponds to an inversion barrier of ∆G = 51 kJ/mol and a
switching rates of kc = 242 and 769 Hz. In comparison to the
original measurements of Wennerströms unsubstituted cage,[16]
the barrier is 14 kJ/mol higher, which can be explained by a further
restriction of movement by the additional ethyl chains. The barrier
of the pyridyl cage 8b is with ∆G = 45 kJ/mol (Tc = 228 K; kc = 99
and 355 Hz) somewhat lower, whereas the methoxy-cage 8c
does not invert in the temperature range between 223 and 323 K.
Indeed, by measurements at variable temperatures
a
coalescence temperature Tc of 238 K was found, which correlates
to an inversion frequency of kc = 222 Hz and an energy of ∆G =
47 kJ/mol. The single-crystal structure of cage 17 is a C2-
symmetric conformer. With the assumption that the para-
substituted phenylene rings (highlighted in blue) can freely rotate
in solution along the 1,4-axis, for the fixed conformer four different
signals for the aromatic protons are expected to resonate, which
is the case at 218 K (δ = 7.32, 6.85, 6.10 and 5.95 ppm; see
Supporting Information).
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a)
a)
b)
b)
Hi
Figure 5. a) Synthesis of C84H84 derivative 17; i) NaBH4, MeOH, reflux, 72 h; ii)
isoamyl nitrite, 50 C, 12 h; iii) Na2S2O4, NaOHaq (20%), EtOH, reflux 12 h. b)
º
Single crystal structures of 17 by X-ray diffraction. Shown is only one conformer
each. Hydrogen atoms and solvate molecules are omitted for clarity. To highlight
the connectivity, alternating rings are colored in blue. For detailed
crystallographic information, see Supporting Information).
In conclusion, we have developed a straightforward synthetic
method to transform imine-cages into hydrocarbon cages in only
three steps, exploiting the high-yielding imine-bond formation in
the first step to generate the cage scaffold. To transform the imine
cages to hydrocarbon cages, these needed to be reduced and
nitrosylated to apply the Overberger reaction to finally form the C-
C bonds. By this method, Wennerström-type cages 8a-8c were
synthesized in 14-24% overall yield, which is ten to twenty times
higher than by the originally one-step method reported before.
The real strength of this method is demonstrated in the four-step
synthesis of a C72H72 and a C84H84 derivative that were previously
unknown. The overall yield for the four-step synthesis of C72H72
derivative 13 for instance, is with 1.4% already higher than those
reported for smaller C60H60 (0.8 %, 20 steps!).[9b] Doubtless,
carbon cages based on alkyne metathesis can be achieved in
much higher yields,[5c] but it does not allow the synthesis of less
symmetric compounds from more than one building block as it is
the case with our method. This gives the opportunity to a larger
variety of compounds. Currently we are re-investigating the
Overberger conditions to finally improve the yields of the carbon
cage formation as well as looking in more detail at the substrate
scope.
Ho1
Ho2
c)
e)
d)
Figure 4. a) Synthesis of ‘cubic’ cage derivative 13; i) NaBH4, MeOH, reflux, 72
h; ii) tert-butyl nitrite, 50 oC, 72 h; iii) Na2S2O4, NaOHaq (20%), EtOH, reflux 72
h. b) 1H NMR of 13 at various temperatures (CDCl3, 400 MHz). c) and d) Single
crystal structures by X-ray diffraction. Shown is only one conformer each.
Hydrogen atoms and solvate molecules are omitted for clarity. To highlight the
connectivity, alternating rings are colored in green. For detailed crystallographic
information, see Supporting Information). e) interconversion of helical
conformers and assignment of aromatic protons as detected at 253 K.
Acknowledgements
The authors like to thank the European Research Council (ERC)
for funding this project in the frame of the consolidators grant
CaTs n DOCs (grant agreement no.725765). We thank Alexandra
Rowse for proof reading.
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Cuthbertson, H. Armstrong, M. E. Briggs, K. E. Jelfs, A. I. Cooper, Nat.
Commun. 2018, 9, 2849.
Keywords: Imine • Organic Cage Compounds • CC bond
formation • Ring Contraction • Hydrocarbons
[19] Crystallographic data for the structure reported in this article have
been deposited at the Cambridge Crystallographic Data Centre,
under deposition no. CCDC 1858590 (13), 1858591 (13) 1858592
(11) 1858593 (5a)1858594 (8a)1858595 (9)1858596 (8b)1858597
(17)1858598 (8c). Copies of the data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif. All other data supporting
the findings of this study are available within the Article and its
Supplementary Information, or from the corresponding author upon
reasonable request.
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Tobias H. G. Schick, Jochen C. Lauer,
Frank Rominger and Michael Mastalerz
Page No. – Page No.
Transformation of thermodynamically
formed Imine Cages to Hydrocarbon
Cages
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